The line, and I see a few others appear to agree.
To over state my reasoning .... a rod's, a rod, and just that ... a 'wippy lever' to toss the line. What kind of rod (or should that be 'rod action?') is a mater of personal taste. And the depth of your wallet.
The reel? A place to store the line that you don't have in the air. As long it's appropriately sized for the fish being targeted, is a $800.00 'something' really much better than a $50.00 Plug. Medalist?
The leader? You have to have 'something' to connect the line to the 'bait,' what ever that may be. And it could just be a straight length of mono ..... (Well, yes it should be at least a three section drop down from butt to tippet. But I know guys who just use a butt section and tippet.)
Now the line, ah yes .. THE line. Ever counted how many types/kinds/finishes/formations/what-ever's are available for you to connect the reel/rod and fly. Hell, I'm not even going to mention how many "flavors" spey rod lines are available. Mind boggling. But when you get 'the match,' that's when the fun begins.
Mis-matched rod/line ....
Total pain in the A$$.
That's my story and I'm sticking to it.